While shopping around for peroxide I stumbled across the stuff in the 1st picture. It was about £4 from Superdrug and is in a cream/paste form and can be brushed straight on. I think the idea is that you can brush it straight onto hair, but I'm not really sure....

Anyhoo, I tested it on a couple of keys off my old PC keyboard. I applied it neat at 4pm and cleaned it all off at about 9am the next day. I put it in my window, but there was no real sun and I'm not sure if my double-glazing filters UV or not

Either way, it worked. The pics show before and after with another key for reference. The keyboard wasn't too yellowed, but as a proof of concept it seemed to be successful. In the last pic you can definitely see the middle, untreated, key is more yellowed. I'm now doing my A600 keyboard (see my upcoming A600 project for more details in mod.hardware).